Integrated printing systems, such as those referred to as “parallel” printing systems and “cluster” printing systems, have been developed in which several marking engines are under the control of a common control system so that a print job may be split among marking engines which are capable of executing the print job. Each marking engine in the integrated system is capable of marking print media, such as paper, with a marking medium such as ink or toner and, in the case of toner, fusing the toner to the print media. The marking engines may differ in that they run at different speeds, print color or monochrome, and/or provide different levels of print quality, higher print quality often being associated with a higher per page cost.
In some integrated systems, two or more marking engines may each execute a portion of the print job. For example, in duplex (two sided) printing, one marking engine may print the first side of each sheet while a second marking engine prints the second side. In simplex (single sided) printing, first and second marking engines may print alternate sheets. In some print jobs, two marking engines may apply marking media to the same side of the sheet, such as for overprinting. The two or more marking engines may be linked by a common paper path to an output device, such as a finisher, where the portions of the executed print job are combined. In the event that one of the marking engines malfunctions, the control system may reroute the print job, or print job portion, designated for that marking engine to another marking engine which is capable of printing the print job, while the malfunctioning marking engine undergoes repair or is replaced. In some cases, the marking engine may be used for printing those print jobs which are not compromised by the malfunction and may undergo repair when not in use.
Such an integrated system with built in redundancy has the advantage of high availability in that the print job is completed, although in some cases, this may be achieved in a less than optimal manner. For example, the job may be printed at a lower output speed, or at higher per page cost, or at a lower print quality, depending on the marking engine(s) that are currently available, the requirements of other print jobs in the queue, and the customer's preferences.
One problem in integrated systems is that the system relies on fault detection by the individual marking engines. Each marking engine is equipped with sensors which detect and log performance characteristics, such as timing of various events, outputs of feedback control loops, toner properties, voltages and currents, developed toner mass on photoreceptors, print quality and registration, environmental conditions, actuator values, and the like. Additionally, the prior systems also record faults and error counter values. A marking engine may detect a fault which did not, in fact, originate in the marking engine. Rather, the fault may have occurred upstream in the common paper path, for example, in an upstream marking engine. The marking engine reports the detected fault to a scheduling component of the printing system. The scheduling system may attribute the detected fault to the reporting marking engine and may make scheduling decisions which are based on the erroneous information.